1. Field Of The Invention
The present invention relates to image scanning apparatus.
2. Description Of The Prior Art
Image scanning is a process of converting an image recorded on a photosensitive sheet, such as photographic film, to an electronic image. Scanning, as such, is commonly employed as a first step in the transmission of an image from one storage medium to another, or in the enhancement or the analysis of the image prior to its transmission.
With a photographic film held in a given plane, an image scanner measures the optical density of the film by illuminating it with light of known intensity and measuring the amount of light transmitted through, or reflected from, the film. In doing so, the scanner effectively divides the image recorded on the film into discrete picture elements, or pixels, and assigns to each a number or value representing an average density for each pixel. Commonly, the pixels are arranged in rows and columns to form a two-dimensional grid with the density of each pixel corresponding to a relatively small portion of the overall image.
There are various image scanning systems known in the art. Such systems are of two basic types, i.e. so-called flat-bed scanning and so-called cylindrical or drum scanning. With film of a large format, e.g. greater than 35 mm, flat-bed scanner apparatus requires a support to maintain the film precisely in a predetermined flat plane. To that end, film is commonly sandwiched tightly between an opposing pair of glass plates. With drum scanner apparatus, a clamping mechanism serves to maintain the film in a curved plane tightly against the surface of the drum. These arrangements, however, suffer from a disadvantage in that the support for the film--glass plates with flat-bed apparatus and the drum with cylindrical apparatus--can create optical interference patterns, known as Newton rings, in the illumination applied to the film.
There are procedures used in the art, with a moderate degree of success, in the eliminating of Newton rings. For example, etched glass, known as anti-Newton glass, can be employed. Such glass, however, is expensive due to its added cost of manufacture. Two other procedures include applying oil to each film sheet, and the use of the so-called "cornstarch method". The former requires that the film support be wiped completely clean of oil each time a different film is to be scanned. Cornstarch works in a hit or miss manner to eliminate Newton rings; thus, that method can be time-consuming in order to obtain fully satisfactory results.
In addition to problems associated with Newton rings, an illumination system of an image scanner should be efficient. It is important, for example from the standpoint of scanning speed, that light not be wasted. That is, light should be directed, to the extent possible, onto only the pixel (or pixels) whose optical density is being measured at that particular time.
The spatial resolution of film determines scene detail--the amount of information--recorded. For example, Ektachrome 64 film, manufactured by Eastman Kodak Company, exposed with an excellent lens can record information to 100 cycles per millimeter (mm). In this case, film pixel pitch should be five microns (.mu.) to extract all information.
However, information of such minute detail --a few wavelengths of light--imposes stringent requirements on an illumination system. It can be appreciated by those skilled in the image scanner art that it may not be possible, particularly with a high-definition film, to concentrate light solely on pixels of interest, to the total exclusion of other pixels. Nevertheless, light should be directed, to the fullest extent possible, onto only the pixel or pixels whose density is being measured. To some extent, the aforementioned glass plates of flat-bed apparatus and the drum of cylindrical apparatus disperse incident light, thereby contributing to inefficiency.
Although unwanted dispersion can be wasteful, specular illumination, i.e. light concentrated in a small solid angle, highlights particles and scratches on the film surface. Diffuse illumination, on the other hand, so long as it is not wasted, is highly preferred since it, contrary to specular light, reduces granularity due to particles on the film surface and hides film scratches. The latter feature is particularly important when the cosmetic quality of the image is critical.
Further desirable characteristics of an image scanner are the absence of electronic flare and the suppression of cross talk. The former arises when light is projected onto pixels distant from the one(s) whose density is being measured; cross talk results from light on a neighboring pixel. Thus, the illumination system of an image scanner is subject to the somewhat conflicting requirements of the need for diffuse illumination while using light in an efficient manner to enhance scanner speed and to reduce flare and cross talk.